Conventionally, a thermoelectric material of Bi.sub.2 Te.sub.3 is known well as a thermoelectric material that makes use of Seebeck effect and Peltier effect, and has been practically employed in some applications. Such material has been limitedly used at about a room temperature, because the operating temperature range thereof is very narrow. In contrast, a thermoelectric CoSb.sub.3 based material has the characteristics that an electron or hole mobility is very high in an intermetallic CoSb.sub.3 based compound presenting Skutterudite structure. It is expected that the material can provide a high thermoelectric conversion efficiency as well as a wide operating temperature range.
Generally, a property important of a thermoelectric material is evaluated by a figure of merit Z=S.sup.2 .sigma./k using Seebeck coefficient S, an electric conductivity .sigma. and a heat conductivity k as parameters. In order to increase the figure of merit Z, it is required that S and .sigma. are higher, and k is lower.
Regarding a thermoelectric material of CoSb.sub.3 in prior art, Japanese Patent Publication No. 8-186294A discloses that power factor S.sup.2 .sigma. is increased by adding Pd, Rh, Ru or the like to CoSb.sub.3, and further points out that S.sup.2 .sigma. is increased in such material, because .sigma. is increased by densifying a sintered body. It is known that a similar effect can be also obtained by alternatively adding Pt.
Said Japanese Patent Publication No. 8-86294A, regarding techniques of manufacturing a sintered body for a thermoelectric material of CoSb.sub.3 system, also discloses a method of sintering compacts of powders which are provided by grinding an ingot form ed by melting Co and Sb, and a method of further densifying the sintered body by hot pressing or HIP after the sintering process.
Although Pd and Pt added to a CoSb.sub.3 based thermoelectric material increases S.sup.2 .sigma., it never causes significant reduction of the heat conductivity k. In order to further increase the figure of merit of the thermoelectric material of CoSb.sub.3, there is still an issue of obtaining a lower heat conductivity. Although the electric conductivity .sigma. can be increased by densifying a sintered body, such sintering process of a long time as conventionally employed causes crystal grains to grow and coarsen. Since the coarsening of crystal grains simultaneously causes increase of the heat conductivity k, the figure of merit has never been increased so much.
In order to use such material for a thermoelectric module, it is required, in terms of the efficiency of power generation, to produce a p-n junction by using two type of materials, that is, p-type and n-type thermoelectric materials of CoSb.sub.3. Addition of either Pd or Pt conventionally conducted represents an n-type thermoelectric material. Although a thermoelectric material of CoSb.sub.3 with a low content of impurities comes to be of p-type itself, and has a high Seebeck effect, it is low in electric conductivity a and insufficient in the figure of merit.
Regarding a p-type thermoelectric material, any element substitutable for Co in CoSb.sub.3 has not been discovered yet, and it is required to find out a appropriately substitutable element. It is generally considered that such element similar in atomic characteristics as transition metals of iron group, such as Mn, Cr, Fe represents a relatively easily substitutable element for Co.
However, substitution of such element for Co has rarely been achieved by such conventional method of alloying it into the CoSb.sub.3 based material. Although addition of the transition metal as Mn, Cr and Fe increases a carrier density, and brings about increase of the electric conductivity .sigma., it tends to cause considerable reduction of the Seebeck coefficient S, hence reduction of the power factor S.sup.2 .sigma..